The present invention pertains to an oscillation circuit, such as an oscillation circuit using a SAW (Surface Acoustic Wave) resonator.
Oscillation circuits using SAW oscillators can restrain phase noise (also known as jitter) to a low level. They can realize excellent phase noise characteristics compared to other oscillation circuits. Also, since SAW resonators exhibit strong mechanical vibration and the oscillation frequency is hardly affected by outside mechanical impact, they are widely used in portable communication terminals, such as mobile phones.
In oscillation circuits using SAW oscillators, however, it is difficult to adjust the structural oscillation frequency of the element. For example, even if the load capacitance of the oscillation circuit using a SAW oscillator changes, the range of variation of its oscillation frequency will be narrow. Consequently, if the oscillation frequency of the SAW oscillator is controlled by adjusting the load capacitance, the controllable frequency range becomes very narrow.
Also, since the oscillation frequency of the SAW oscillator changes significantly with ambient temperature, it is necessary to use a temperature-compensation circuit in order to provide a stable oscillation signal.
The objective of the present invention is to solve the problems by providing an oscillation circuit that uses a SAW oscillator, which is able to control the oscillation frequency easily and correct the temperature characteristic of the oscillator so that a stable oscillation signal unaffected by high temperatures can be generated.
In order to realize the objective, the first oscillation circuit of the present invention comprises an oscillator that outputs an oscillation signal, a frequency divider that frequency-divides the oscillation signal at a prescribed frequency division value and outputs a frequency-divided signal, a data retention circuit that receives input data corresponding to the frequency-divided signal and retains the input data, an adder that adds the retained data output from the data retention circuit to a prescribed addition value and outputs the result to the data retention circuit, a quadrature signal generating circuit that outputs first and second signals in quadrature corresponding to the retained data, and a modulation circuit that modulates the oscillation signal corresponding to the first and second signals and outputs an output signal shifted by as much as the frequency of the quadrature signal.
The second oscillation circuit of the present invention comprises an oscillator that outputs an oscillation signal, a frequency divider that frequency-divides the oscillation signal at a prescribed frequency division value and outputs a frequency-divided signal, a data retention circuit that receives input data corresponding to the frequency-divided signal and retains the input data, a temperature sensor for detecting temperature, a correction data generating circuit that generates correction data with respect to the oscillation signal corresponding to the temperature detected by the temperature sensor, an adder that adds the retained data output from the data retention circuit to the correction data and outputs the result to the data retention circuit, a quadrature signal generating circuit that outputs first and second signals in quadrature corresponding to the retained data, and a modulation circuit that modulates the oscillation signal corresponding to the first and second signals and outputs an output signal shifted by as much as the frequency of the quadrature signal.
Also, in the present invention, the adder may output the sum except for the carry signal to the data retention circuit.
In addition, in the present invention, the modulation circuit phase may modulate the oscillation signal corresponding to the first and second signals.
Moreover, in the present invention, the oscillator may be a SAW oscillator, and the modulation circuit may be an IQ modulator.